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package java.lang.invoke;

import sun.invoke.empty.Empty;

import static java.lang.invoke.MethodHandleStatics.*;
import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP;

/**
 * A {@code CallSite} is a holder for a variable {@link MethodHandle}, which is called its {@code
 * target}. An {@code invokedynamic} instruction linked to a {@code CallSite} delegates all calls to
 * the site's current target. A {@code CallSite} may be associated with several {@code
 * invokedynamic} instructions, or it may be "free floating", associated with none. In any case, it
 * may be invoked through an associated method handle called its {@linkplain #dynamicInvoker dynamic
 * invoker}. <p> {@code CallSite} is an abstract class which does not allow direct subclassing by
 * users.  It has three immediate, concrete subclasses that may be either instantiated or
 * subclassed. <ul> <li>If a mutable target is not required, an {@code invokedynamic} instruction
 * may be permanently bound by means of a {@linkplain ConstantCallSite constant call site}. <li>If a
 * mutable target is required which has volatile variable semantics, because updates to the target
 * must be immediately and reliably witnessed by other threads, a {@linkplain VolatileCallSite
 * volatile call site} may be used. <li>Otherwise, if a mutable target is required, a {@linkplain
 * MutableCallSite mutable call site} may be used. </ul> <p> A non-constant call site may be
 * <em>relinked</em> by changing its target. The new target must have the same {@linkplain
 * MethodHandle#type() type} as the previous target. Thus, though a call site can be relinked to a
 * series of successive targets, it cannot change its type. <p> Here is a sample use of call sites
 * and bootstrap methods which links every dynamic call site to print its arguments:
 * <blockquote><pre>{@code
 * static void test() throws Throwable {
 * // THE FOLLOWING LINE IS PSEUDOCODE FOR A JVM INSTRUCTION
 * InvokeDynamic[#bootstrapDynamic].baz("baz arg", 2, 3.14);
 * }
 * private static void printArgs(Object... args) {
 * System.out.println(java.util.Arrays.deepToString(args));
 * }
 * private static final MethodHandle printArgs;
 * static {
 * MethodHandles.Lookup lookup = MethodHandles.lookup();
 * Class thisClass = lookup.lookupClass();  // (who am I?)
 * printArgs = lookup.findStatic(thisClass,
 * "printArgs", MethodType.methodType(void.class, Object[].class));
 * }
 * private static CallSite bootstrapDynamic(MethodHandles.Lookup caller, String name, MethodType
 * type) {
 * // ignore caller and name, but match the type:
 * return new ConstantCallSite(printArgs.asType(type));
 * }
 * }</pre></blockquote>
 *
 * @author John Rose, JSR 292 EG
 */
abstract
public class CallSite {

  static {
    MethodHandleImpl.initStatics();
  }

  // The actual payload of this call site:
    /*package-private*/
  MethodHandle target;    // Note: This field is known to the JVM.  Do not change.

  /**
   * Make a blank call site object with the given method type.
   * An initial target method is supplied which will throw
   * an {@link IllegalStateException} if called.
   * <p>
   * Before this {@code CallSite} object is returned from a bootstrap method,
   * it is usually provided with a more useful target method,
   * via a call to {@link CallSite#setTarget(MethodHandle) setTarget}.
   *
   * @throws NullPointerException if the proposed type is null
   */
    /*package-private*/
  CallSite(MethodType type) {
    target = makeUninitializedCallSite(type);
  }

  /**
   * Make a call site object equipped with an initial target method handle.
   *
   * @param target the method handle which will be the initial target of the call site
   * @throws NullPointerException if the proposed target is null
   */
    /*package-private*/
  CallSite(MethodHandle target) {
    target.type();  // null check
    this.target = target;
  }

  /**
   * Make a call site object equipped with an initial target method handle.
   *
   * @param targetType the desired type of the call site
   * @param createTargetHook a hook which will bind the call site to the target method handle
   * @throws WrongMethodTypeException if the hook cannot be invoked on the required arguments, or if
   * the target returned by the hook is not of the given {@code targetType}
   * @throws NullPointerException if the hook returns a null value
   * @throws ClassCastException if the hook returns something other than a {@code MethodHandle}
   * @throws Throwable anything else thrown by the hook function
   */
    /*package-private*/
  CallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {
    this(targetType);
    ConstantCallSite selfCCS = (ConstantCallSite) this;
    MethodHandle boundTarget = (MethodHandle) createTargetHook.invokeWithArguments(selfCCS);
    checkTargetChange(this.target, boundTarget);
    this.target = boundTarget;
  }

  /**
   * Returns the type of this call site's target. Although targets may change, any call site's type
   * is permanent, and can never change to an unequal type. The {@code setTarget} method enforces
   * this invariant by refusing any new target that does not have the previous target's type.
   *
   * @return the type of the current target, which is also the type of any future target
   */
  public MethodType type() {
    // warning:  do not call getTarget here, because CCS.getTarget can throw IllegalStateException
    return target.type();
  }

  /**
   * Returns the target method of the call site, according to the
   * behavior defined by this call site's specific class.
   * The immediate subclasses of {@code CallSite} document the
   * class-specific behaviors of this method.
   *
   * @return the current linkage state of the call site, its target method handle
   * @see ConstantCallSite
   * @see VolatileCallSite
   * @see #setTarget
   * @see ConstantCallSite#getTarget
   * @see MutableCallSite#getTarget
   * @see VolatileCallSite#getTarget
   */
  public abstract MethodHandle getTarget();

  /**
   * Updates the target method of this call site, according to the
   * behavior defined by this call site's specific class.
   * The immediate subclasses of {@code CallSite} document the
   * class-specific behaviors of this method.
   * <p>
   * The type of the new target must be {@linkplain MethodType#equals equal to}
   * the type of the old target.
   *
   * @param newTarget the new target
   * @throws NullPointerException if the proposed new target is null
   * @throws WrongMethodTypeException if the proposed new target has a method type that differs from
   * the previous target
   * @see CallSite#getTarget
   * @see ConstantCallSite#setTarget
   * @see MutableCallSite#setTarget
   * @see VolatileCallSite#setTarget
   */
  public abstract void setTarget(MethodHandle newTarget);

  void checkTargetChange(MethodHandle oldTarget, MethodHandle newTarget) {
    MethodType oldType = oldTarget.type();
    MethodType newType = newTarget.type();  // null check!
    if (!newType.equals(oldType)) {
      throw wrongTargetType(newTarget, oldType);
    }
  }

  private static WrongMethodTypeException wrongTargetType(MethodHandle target, MethodType type) {
    return new WrongMethodTypeException(String.valueOf(target) + " should be of type " + type);
  }

  /**
   * Produces a method handle equivalent to an invokedynamic instruction which has been linked to
   * this call site. <p> This method is equivalent to the following code:
   * <blockquote><pre>{@code
   * MethodHandle getTarget, invoker, result;
   * getTarget = MethodHandles.publicLookup().bind(this, "getTarget",
   * MethodType.methodType(MethodHandle.class));
   * invoker = MethodHandles.exactInvoker(this.type());
   * result = MethodHandles.foldArguments(invoker, getTarget)
   * }</pre></blockquote>
   *
   * @return a method handle which always invokes this call site's current target
   */
  public abstract MethodHandle dynamicInvoker();

  /*non-public*/ MethodHandle makeDynamicInvoker() {
    MethodHandle getTarget = GET_TARGET.bindArgumentL(0, this);
    MethodHandle invoker = MethodHandles.exactInvoker(this.type());
    return MethodHandles.foldArguments(invoker, getTarget);
  }

  private static final MethodHandle GET_TARGET;
  private static final MethodHandle THROW_UCS;

  static {
    try {
      GET_TARGET = IMPL_LOOKUP.
          findVirtual(CallSite.class, "getTarget", MethodType.methodType(MethodHandle.class));
      THROW_UCS = IMPL_LOOKUP.
          findStatic(CallSite.class, "uninitializedCallSite",
              MethodType.methodType(Object.class, Object[].class));
    } catch (ReflectiveOperationException e) {
      throw newInternalError(e);
    }
  }

  /**
   * This guy is rolled into the default target if a MethodType is supplied to the constructor.
   */
  private static Object uninitializedCallSite(Object... ignore) {
    throw new IllegalStateException("uninitialized call site");
  }

  private MethodHandle makeUninitializedCallSite(MethodType targetType) {
    MethodType basicType = targetType.basicType();
    MethodHandle invoker = basicType.form().cachedMethodHandle(MethodTypeForm.MH_UNINIT_CS);
    if (invoker == null) {
      invoker = THROW_UCS.asType(basicType);
      invoker = basicType.form().setCachedMethodHandle(MethodTypeForm.MH_UNINIT_CS, invoker);
    }
    // unchecked view is OK since no values will be received or returned
    return invoker.viewAsType(targetType, false);
  }

  // unsafe stuff:
  private static final long TARGET_OFFSET;

  static {
    try {
      TARGET_OFFSET = UNSAFE.objectFieldOffset(CallSite.class.getDeclaredField("target"));
    } catch (Exception ex) {
      throw new Error(ex);
    }
  }

  /*package-private*/
  void setTargetNormal(MethodHandle newTarget) {
    MethodHandleNatives.setCallSiteTargetNormal(this, newTarget);
  }

  /*package-private*/
  MethodHandle getTargetVolatile() {
    return (MethodHandle) UNSAFE.getObjectVolatile(this, TARGET_OFFSET);
  }

  /*package-private*/
  void setTargetVolatile(MethodHandle newTarget) {
    MethodHandleNatives.setCallSiteTargetVolatile(this, newTarget);
  }

  // this implements the upcall from the JVM, MethodHandleNatives.makeDynamicCallSite:
  static CallSite makeSite(MethodHandle bootstrapMethod,
      // Callee information:
      String name, MethodType type,
      // Extra arguments for BSM, if any:
      Object info,
      // Caller information:
      Class<?> callerClass) {
    MethodHandles.Lookup caller = IMPL_LOOKUP.in(callerClass);
    CallSite site;
    try {
      Object binding;
      info = maybeReBox(info);
      if (info == null) {
        binding = bootstrapMethod.invoke(caller, name, type);
      } else if (!info.getClass().isArray()) {
        binding = bootstrapMethod.invoke(caller, name, type, info);
      } else {
        Object[] argv = (Object[]) info;
        maybeReBoxElements(argv);
        switch (argv.length) {
          case 0:
            binding = bootstrapMethod.invoke(caller, name, type);
            break;
          case 1:
            binding = bootstrapMethod.invoke(caller, name, type,
                argv[0]);
            break;
          case 2:
            binding = bootstrapMethod.invoke(caller, name, type,
                argv[0], argv[1]);
            break;
          case 3:
            binding = bootstrapMethod.invoke(caller, name, type,
                argv[0], argv[1], argv[2]);
            break;
          case 4:
            binding = bootstrapMethod.invoke(caller, name, type,
                argv[0], argv[1], argv[2], argv[3]);
            break;
          case 5:
            binding = bootstrapMethod.invoke(caller, name, type,
                argv[0], argv[1], argv[2], argv[3], argv[4]);
            break;
          case 6:
            binding = bootstrapMethod.invoke(caller, name, type,
                argv[0], argv[1], argv[2], argv[3], argv[4], argv[5]);
            break;
          default:
            final int NON_SPREAD_ARG_COUNT = 3;  // (caller, name, type)
            if (NON_SPREAD_ARG_COUNT + argv.length > MethodType.MAX_MH_ARITY) {
              throw new BootstrapMethodError("too many bootstrap method arguments");
            }
            MethodType bsmType = bootstrapMethod.type();
            MethodType invocationType = MethodType
                .genericMethodType(NON_SPREAD_ARG_COUNT + argv.length);
            MethodHandle typedBSM = bootstrapMethod.asType(invocationType);
            MethodHandle spreader = invocationType.invokers().spreadInvoker(NON_SPREAD_ARG_COUNT);
            binding = spreader
                .invokeExact(typedBSM, (Object) caller, (Object) name, (Object) type, argv);
        }
      }
      //System.out.println("BSM for "+name+type+" => "+binding);
      if (binding instanceof CallSite) {
        site = (CallSite) binding;
      } else {
        throw new ClassCastException("bootstrap method failed to produce a CallSite");
      }
      if (!site.getTarget().type().equals(type)) {
        throw wrongTargetType(site.getTarget(), type);
      }
    } catch (Throwable ex) {
      BootstrapMethodError bex;
      if (ex instanceof BootstrapMethodError) {
        bex = (BootstrapMethodError) ex;
      } else {
        bex = new BootstrapMethodError("call site initialization exception", ex);
      }
      throw bex;
    }
    return site;
  }

  private static Object maybeReBox(Object x) {
    if (x instanceof Integer) {
      int xi = (int) x;
      if (xi == (byte) xi) {
        x = xi;  // must rebox; see JLS 5.1.7
      }
    }
    return x;
  }

  private static void maybeReBoxElements(Object[] xa) {
    for (int i = 0; i < xa.length; i++) {
      xa[i] = maybeReBox(xa[i]);
    }
  }
}
